Approximately 20% of the 400,000 radiation therapy patients per year have their treatments prescribed according to computer-assisted treatment planning systems. Poorly understood accuracy limitations for these complex systems can lead to seriously incorrect dose delivery and consequent uncontrolled tumor growth or complications due to excessive radiation. The Computer Applications Task Group of the American Association of Physicists in Medicine (AAPM) Radiation Therapy Committee has developed an approach which can provide a versatile method for testing the accuracy of computer systems employed for external photon beam treatment planning dosimetry. The proposed verification system is a self-sufficient test package that can be implemented by any facility which has a treatment planning computer system. The verification package will contain experimental data describing treatment beams and a progressive series of clinically relevant computer test cases capable of isolating specific dosimetric problem areas. Thus, physicists and radiation oncologists can develop an understanding of accuracy limiting situations and provide appropriate alternatives. The test package will be generated from experimental measurements under close Task Group control. The verification system will then be field tested and its effectiveness demonstrated at 12 separate clinical facilities by Task Group members and associates. These tests will demonstrate the effectiveness and flexibility of the test package on a wide variety of treatment planning systems at various clinical facilities. Members of the AAPM represent the professional group most concerned with dosimetry computation, and experts within this organization are in agreement on the proposed approach. The AAPM also maintains established communication and publication mechanisms which will promote broad implementation of the program.